Method for making an improved solid polymer electrolyte electrode using a liquid or solvent

ABSTRACT

The invention is a method for forming a solid polymer electrolyte structure comprising: 
     a. forming a suspension of catalytically active, electrically conductive particles and a liquid such as dibromotetrafluoroethane; 
     b. applying the suspension to at least one side of a fluorocarbon membrane sheet, while said sheet is in its thermoplastic form; 
     c. removing substantially all of the liquid, leaving the particles on the membrane sheet; 
     d. pressing at least a portion of the particles into the membrane sheet; 
     e. contacting the side of the so-treated membrane having the particles on the surface with an electrically conductive, hydraulically permeable matrix; 
     f. subjecting the membrane/matrix combination to a pressure sufficient to embed at least a portion of the matrix into the membrane.

BACKGROUND OF THE INVENTION

Solid polymer electrolyte (SPE) cells refer to cells in which one orboth electrodes are bonded to or embedded in a polymeric ion exchangemembrane. Such cells are rather well known in the art and are discussedin detail in the following U.S. Pat. Nos.: 4,315,805 "Solid PolymerElectrolyte Chlor-Alkali Process", Darlington, et al. (Feb. 16, 1982);4,364,815 "Solid Polymer Electrolyte Chlor-Alkali Process andElectrolytic Cell", Darlington, et al. (Dec. 12, 1982); 4,272,353"Method of Making Solid Polymer Electrolyte Catalytic Electrodes andElectrodes Made Thereby", Lawrence, et al. (June 9, 1981); and 4,394,229"Cathode Element For Solid Polymer Electrolyte", Korach (July 19, 1983).

In SPE cells, a current collector is pressed against and contacts theelectrode and provides a pathway for electrical current to flow from apower supply to the electrode. Current collectors are electricallyconductive, hydraulically permeable matrices which may take a variety ofshapes, sizes, and types, including metallic window screen, punchedmetallic plates, expanded metals, and the like. The following patentsdescribe some commonly-used types of current collectors: U.S. Pat. Nos.4,299,674 "Process For Electrolyzing An Alkali Metal Halide Using ASolid Polymer Electrolyte Cell", Korach (Nov. 10, 1981); 4,468,311"Electrolysis Cell", de Nora, et al. (Aug. 28, 1984); and 4,215,183 "WetProofed Conductive Current Collectors for the Electrochemical Cells",MacLeod (July 29, 1980).

SPE cells often have major problems due to the high electricalresistance between the embedded or bonded electrodes and the currentcollectors which are pressed against the electrode. Many workers in theart have attempted to solve the high resistance problem in a variety ofways. Some solutions include the use of a mattress as shown in U.S. Pat.No. 4,468,311 "Electrolysis Cell", de Nora, et al. (Aug. 28, 1984);applying the electrocatalyst directly to a conductive carbon cloth whichacts as the current collector as shown in U.S. Pat. No. 4,239,396 "ThisCarbon-Cloth-Based Electrocatalytic Gas Diffusion Electrodes, AndElectrochemical Cells Comprising the Same", Allen, et al. (Oct. 6,1981).

The present invention provides an SPE structure that minimizes theelectrical resistance between the current collector and the embedded orbonded electrode.

SUMMARY OF THE INVENTION

The invention is a method for forming a solid polymer electrolytestructure comprising:

a. forming a suspension of catalytically active, electrically conductiveparticles and a liquid such as dibromotetrafluoroethane;

b. applying the suspension to at least one side of a fluorocarbonmembrane sheet, while said sheet is in its thermoplastic form;

c. removing substantially all of the liquid, leaving the particles onthe membrane sheet;

d. pressing at least a portion of the particles into the membrane sheet;

e. contacting the side of the so-treated membrane having the particleson its surface with an electrically conductive, hydraulically permeablematrix;

f. subjecting the membrane/matrix combination to a pressure sufficientto embed at least a portion of the matrix into the membrane.

BRIEF DESCRIPTION OF THE FIGURE

The FIGURE illustrates the SPE structure of the present invention andshows the membrane sheet 120, the plurality of electrically conductiveparticles 110, and the electrically conductive, hydraulically permeablematrix 130.

DETAILED DESCRIPTION OF THE INVENTION

As a result of the intimate contact between the membrane sheet, theelectrically conductive particles, and the electrically conductive,hydraulically permeable matrix (which serves as a current collector andis connected to a power supply), the resistance to the flow ofelectrical energy is minimized and, thus, the cell operates moreefficiently than cells employing the SPE structures of the prior art.

The SPE structure of the present invention includes embodiments whereelectrically conductive particles are bonded to or embedded in one, orboth, sides of the membrane sheet.

The FIGURE shows the SPE structure 100. It is composed of a membranesheet 120 which has a plurality of electrically conductive particlesembedded into it. The particles are in physical and electrical contactwith an electrically conductive, hydraulically permeable matrix 130,which is also embedded into the membrane sheet 120.

The membrane sheet divides the anode compartment from the cathodecompartment and limits the type and amount of fluids and/or ions thatpass between the anode compartment and the cathode compartments. Themembrane may be a single layer membrane or a composite layer membrane.

The membrane may be constructed of a fluorocarbon-type material or of ahydrocarbon-type material. Such membrane materials are well known in theart. Preferably, however, fluorocarbon materials are generally preferredbecause of their chemical stability.

Non-ionic (thermoplastic) forms of perfluorinated polymers described inthe following patents are suitable for use in the present invention:U.S. Pat. Nos. 3,282,875; 3,909,378; 4,025,405; 4,065,366; 4,116,888;4,123,336; 4,126,588; 4,151,052; 4,176,215; 4,178,218; 4,192,725;4,209,635; 4,212,713; 4,251,333; 4,270,996; 4,329,435; 4,330,654;4,337,137; 4,337,211; 4,340,680; 4,357,218; 4,358,412; 4,358,545;4,417,969; 4,462,877; 4,470,889; and 4,478,695; European PatentApplication No. 0,027,009. Such polymers usually have equivalent weightin the range of from about 500 to about 2000.

To allow the cloth and the electrically conductive particles to beembedded into the fluorocarbon membrane, it is desirable for thefluorocarbon membrane to be in its thermoplastic form. It is in athermoplastic form when it is made and before it is converted into anion exchange form. By thermoplastic form, it is meant, for instance,that the membrane has SO₂ X pendant groups rather than ionically bondedSO₃ Na or SO₃ H pendant groups, where X is --F, --CO₂, --CH₃, or aquaternary amine.

Particularly preferred fluorocarbon materials for use in formingmembranes are copolymers of monomer I with monomer II (as definedbelow). Optionally, a third type of monomer may be copolymerized with Iand II.

The first type of monomer is represented by the general formula:

    CF.sub.2 =CZZ'                                             (I)

where:

Z and Z' are independently selected from the group consisting of --H,--Cl, --F, or --CF₃.

The second monomer consists of one or more monomers selected fromcompounds represented by the general formula:

    Y--(CF.sub.2).sub.a --(CFR.sub.f).sub.b --(CFR.sub.f').sub.c --O--[CF(CF.sub.2 X)--CF.sub.2 --O].sub.n --(CF=CF.sub.2  (II)

where:

Y is selected from the group consisting of --SO₂ Z, --CN, --COZ, andC(R³ f)(R⁴ f)OH;

Z is I, Br, Cl, F, OR, or NR₁ R₂ ;

R is a branched or linear alkyl radical having from 1 to about 10 carbonatoms or an aryl radical;

R³ f and R⁴ f are independently selected from the group consisting ofperfluoroalkyl radicals having from 1 to about 10 carbon atoms;

R₁ and R₂ are independently selected from the group consisting of H, abranched or linear alkyl radical having from 1 to about 10 carbon atomsor an aryl radical;

a is 0-6;

b is 0-6;

c is 0 or 1;

provided a+b+c is not equal to 0;

X is Cl, Br, F, or mixtures thereof when n>1;

n is 0 to 6; and

R_(f) and R_(f') are independently selected from the group consisting ofF, Cl, perfluoroalkyl radicals having from 1 to about 10 carbon atomsand fluorochloroalkyl radicals having from 1 to about 10 carbon atoms.

Particularly preferred is when Y is --SO₂ F or --COOCH₃ ; n is 0 or 1;R_(f) and R_(f') are F; X is Cl or F; and a+b+c is 2 or 3.

The third and optional monomer suitable is one or more monomers selectedfrom the compounds represented by the general formula:

    Y'--(CF.sub.2).sub.a' --(CFR.sub.f).sub.b' --(CFR.sub.f').sub.c' --O--[CF(CF.sub.2 X')--CF.sub.2 --O].sub.n' --CF=CF.sub.2 (III)

where:

Y' is F, Cl or Br;

a' and b' are independently 0-3;

c is 0 or 1;

provided a'+b'+c' is not equal to 0;

n' is 0-6;

R_(f) and R_(f') are independently selected from the group consisting ofBr, Cl, F, perfluoroalkyl radicals having from about 1 to about 10carbon atoms, and chloroperfluoroalkyl radicals having from 1 to about10 carbon atoms; and

X' is F, Cl, Br, or mixtures thereof when n'>1.

Conversion of Y to ion exchange groups is well known in the art andconsists of reaction with an alkaline solution.

While the fluorocarbon membrane is in its thermoplastic form, it iscapable of softening when heated and hardening again when cooled. Thus,the cloth can be easily pressed into the fluorocarbon membrane when thefluorocarbon membrane has been heated. The temperature to which thefluorocarbon membrane is preferably heated to make it sufficiently softto allow the cloth to be embedded therein depends, to a great extent, onthe chemical formulation of the fluorocarbon membrane. In general,however, temperatures in the range of from about 150° Celsius to about350° Celsius for fluorocarbon membranes having Y=--SO₂ F (as defined inEquation II above), or 150° Celsius to 300° Celsius for fluorocarbonmembranes having Y=--CO₂ CH₃ (as defined in Equation II above).Hydrocarbon-based membranes may (depending upon the exact composition ofthe hydrocarbon material) be heated from about 100° Celsius to about190° Celsius.

For example, a membrane sheet may be prepared by hot pressing a sulfonylfluoride powder having an equivalent weight of about 1000, as describedin U.S. Pat. No. 4,330,654 between two sheets of glass reinforcedpolytetrafluoroethylene at a temperature of about 310° Celsius under apressure of about 0.75 tons per square inch for about 1.25 minutes. Theresulting 6-7 inch diameter sheet is preferably in the range of fromabout 0.0001 to about 0.010 inches thick. More preferably, the thicknessof the sheet is from about 0.0005 to about 0.015 inches thick. Mostpreferably, the thickness of the sheet is from about 0.002 to about 0.06inches thick.

In the present invention, it is important to make an effective bondbetween the electrically conductive, hydraulically permeable matrix andthe membrane. Such a bond may be made with or without the use ofexternally-applied pressure during bonding. It has been discovered,however, that better bonding is generally obtained when the membrane andthe electrically conductive, hydraulically permeable matrix are firstcontacted and heated at zero pressure for about 1 minute, followed bypressing at about 1 to about 8 tons per square inch for from about 0.2to about 2 minutes.

The present invention requires that at least one of the electrodes be inthe form of a plurality of electrically conductive particles embeddedinto the membrane sheet. This is what makes a SPE electrode. Theelectrode composed of a plurality of electrically conductive particlescan be either the cathode or the anode. Optionally, both electrodes canbe electrically conductive particles embedded into opposite sides of themembrane sheet. For the purposes of the present discussion, the forms ofboth electrodes will be described as though they are electricallyconductive particles and will also be described as if they are separate,conventional electrodes.

Conventional anodes are usually hydraulically permeable, electricallyconductive structures made in a variety of shapes and styles including,for example, a sheet of expanded metal, perforated plate, punched plate,unflattened diamond shaped expanded metal, or woven metallic wire.Metals suitable for use as anodes include tantalum, tungsten, columbium,zirconium, molybdenum, and preferably, titanium and alloys containingmajor amounts of these metals.

Optionally the anodes may be a SPE electrode consisting of a pluralityof electrically conductive particles embedded into the membrane sheet.Materials suitable for use as electrocatalytically active anodematerials include, for example, activating substances such as oxides ofplatinum group metals like ruthenium, iridium, rhodium, platinum,palladium, either alone or in combination with an oxide of afilm-forming metal. Other suitable activating oxides include cobaltoxide either alone or in combination with other metal oxides. Examplesof such activating oxides are found in U.S. Pat. Nos. 3,632,498;4,142,005; 4,061,549; and 4,214,971.

Conventional cathodes are usually hydraulically permeable, electricallyconductive structures made in a variety of shapes and styles including,for example, a sheet of expanded metal, perforated plate, punched plate,unflattened diamond shaped expanded metal, or woven metallic wire.Metals suitable for use as cathode include, for example, copper, iron,nickel, lead, molybdenum, cobalt, alloys including major amounts ofthese metals, such as low carbon stainless steel, and metals or alloyscoated with substances such as silver, gold, platinum, ruthenium,palladium, and rhodium.

Optionally, as has been stated, the cathode may be an SPE electrodeconsisting of a plurality of electrically conductive particles embeddedinto the membrane sheet. Materials suitable for use aselectrocatalytically active cathode materials include, for example,platinum group metal or metal oxide, such as ruthenium or rutheniumoxide. U.S. Pat. No. 4,465,580 describes such cathodes.

The electrically conductive particles, whether used as an anode or as acathode are preferably finely divided and have a high surface area. Forexample, in the case of an oxygen or hydrogen electrode fuel cell,platinum black (surface area greater than 25 m² /gram) or high surfacearea (800-1800 m² /g) platinum on activated carbon powder (averageparticle size 10-30 microns) are quite suitable for use as the anode andthe cathode. In the case of a chlorine cell, an may be prepared in whichruthenium dioxide particles are prepared by thermal decomposition ofruthenium nitrate for 2 hours at 450° Celsius. The resulting oxide maythen be ground using a mortar and pestle and the portion which passedthrough a 325 mesh sieve (less than 44 microns) used to prepare anelectrode.

The electrically conductive, hydraulically permeable matrix which actsas a current collector to transmit electrical energy to or from the SPEelectrode, may be composed of a variety of substances including carboncloth, carbon paper, carbon felt, metallic screens, metallic felt, andporous metallic sheets. Preferably, however, the electricallyconductive, hydraulically permeable matrix is a carbon cloth becausecarbon cloth is readily available, performs well, is easily handled, andis relatively inexpensive.

The cloth most preferably used in this invention is one having lowelectrical resistivity, relatively inexpensive, possess sufficientstrength for fabrication, and have adequate surface properties, such asroughness, to provide good bonding between the ion exchange membrane anditself. It is also preferable to provide good electrical contact betweenthe carbon cloth and the electrocatalytically active particles of theelectrode.

The type of carbon cloth suitable for use in the present invention iscommercially available from a variety of sources including: StackpoleFibers Co. sold under the names Panex PWB-3, PWB-6, KFB and SWB-8; fromUnion Carbide Corp. sold under the names WCA Graphite Cloth and VCK andVCA carbon cloth. Carbon cloth may also be woven from carbon fibersavailable from Fiberite Corp. sold under the names Celion 1000, Celion3000, Celion 6000, Celion 12000, or from Celanese Corporation sold asC-6, or G-50. These materials may vary in physical properties but areacceptable for use in the present invention if they are sufficientlystrong to maintain their physical integrity during fabrication. Fibersize and weave patterns may also vary and are not critical to thesuccessful operation of the present invention. Cloth useful in thepresent invention preferably has a thickness of from about 0.002 inchesto about 0.025 inches and have electrical resistivities of from about600,000 to about 1375 microohm-centimeters. More preferably the clothused in the present invention has a resistivity of approximately 1500microohm-centimeters.

The SPE structure may then be fabricated by preparing the membrane inthe thermoplastic form, embedding the electrocatalytically activeparticles into the membrane, bonding the current collector over theparticles, and then converting the membrane to its ionic form byreacting it with, in the case of --SO₂ F pendant groups, 25 weight %NaOH under the following conditions: 1. immerse the film in about 25weight percent sodium hydroxide for about 16 hours at a temperature ofabout 90° Celsius; 2. rinse the film twice in deionized water heated toabout 90° Celsius, using about 30 to about 60 minutes per rinse. Thependant group is then in the --SO₃ ⁻ Na⁺ form. Cations other than Na⁺can be made to replace the Na⁺ if practical (such as H⁺).

The electrocatalytically active particles may be incorporated into thesurface of the membrane using a variety of techniques including, makinga suspension of the particles with a liquid and spraying or pouring thesuspension over the membrane, removing the solvent, for example byallowing the liquid to evaporate, and then hot pressing the particlesinto the membrane with or without the carbon cloth in place. For exampleplatinum and carbon particles may be slurried indibromotetrafluoroethane and poured or sprayed onto a membrane. Thedibromotetrafluoroethane is then allowed to evaporate. Carbon clothcurrent collectors can then be hot pressed onto these so-formedelectrodes.

The quantity of particles used on the membrane film to form the SPEelectrode may vary depending upon the activity of the electrocatalyst,its cost, etc. For chlor-alkali SPE membranes, the amount of catalystused is usually from about 0.4 to about 1.0 milligrams catalyst/squarecentimeter of membrane. There is an upper limit on the amount ofparticles which may be placed onto the membrane because the particlespenetrate the membrane. The upper limit has been determined to be about25 milligrams catalyst/square centimeter of membrane.

The solid polymer electrolyte structure of the present invention isuseful in a wide variety of electrochemical cells including, forexample, fuel cells for the continuous production of electrical energy;electrolysis cells for the production of chemical products; andbatteries for the intermittent production of electrical energy.

We claim:
 1. A method for forming a solid polymer electrolyte structureconsisting essentially of:a. forming a suspension of catalyticallyactive, electrically conductive particles and a liquid; b. applying thesuspension to at least one side of a fluorcarbon membrane sheet, whilesaid sheet is in its thermoplastic form; c. removing substantially allof the liquid, leaving the particles on the membrane sheet; d. pressingat least a portion of the particles into the membrane sheet; e.contacting the side of the so-treated membrane having the particles onthe surface with an electrically conductive, hydraulically permeablematrix; f. subjecting the membrane/matrix combination to a pressuresufficient to bond at least a portion of the matrix to the membrane. 2.The method of claim 1 wherein the membrane is from about 0.0005 to about0.015 inches thick.
 3. The method of claim 1 wherein the particles areapplied to both sides of the membrane.
 4. The method of claim 1 whereinthe matrix is selected from the group consisting of carbon cloth, carbonpaper. carbon felt, metallic screen, metallic felt and a porous metallicsheet.
 5. The method of claim 4 wherein the carbon cloth has a thicknessof from about 0.002 to about 0.025 inches.
 6. The method of claim 1wherein the matrix is carbon paper.
 7. The method of claim 1 wherein thematrix has a resistivity of from about 600,000 to about 1375microohm-centimeters.
 8. The method of claim 1 wherein the matrix has aresistivity of about 1500 microohm-centimeters.
 9. The method of claim 1wherein the catalytically active particles have an average particle sizediameter of from about 10 to about 30 microns.
 10. The method of claim 1wherein the catalytically active particles have a surface area of fromabout 800 to about 1800 square meters per gram.
 11. The method of claim1 wherein the catalytically active particles are selected from the groupconsisting of platinum group metals, platinum group metal oxides,ruthenium, iridium, rhodium, platinum, palladium, either alone or incombination with an oxide of a film-forming metal, and cobalt oxideeither alone or in combination with other metal oxides.
 12. The methodof claim 1 wherein the catalytically active particles are present on themembrane at a level of less than about 25 milligrams per squarecentimeter of membrane.
 13. The method of claim 1 wherein thecatalytically active particles are present on the membrane at a level offrom about 0.4 to about 1.0 milligrams per square centimeter ofmembrane.
 14. The method of claim 1 wherein the fluorocarbon polymer hasan equivalent weight of from about 500 to about
 2000. 15. The method ofclaim 1 wherein the plurality of electrically conductive particlesconstitute an anode.
 16. The method of claim 1 wherein the plurality ofelectrically conductive particles constitute an cathode.
 17. The methodof claim 1 wherein a plurality of electrically conductive particles forman anode electrode on one side of the membrane sheet and a plurality ofelectrically conductive particles form a cathode electrode on theopposite side of the membrane.
 18. The method of claim 17 wherein oneelectrode is composed of a plurality of electrically conductiveparticles and the other electrode is composed of a porous metal plate.19. The method of claim 1 wherein the liquid isdibromotetrafluoroethane.